Well, at least one envelope from Mead's Mills exists. I haven't seen any with its postmark but one will eventually surface somewhere. I'm not sure if J. T. Whiting sent it or if Ramsdell received the letter in care of him.
I bought this envelope today with an Eloise postmark of February 7, 1948. It's addressed to the widow of Judge Charles L. Fifield of Janesville, Wisconson. Unfortunately, there's no return address or letter inside.